This invention is concerned with the economic conversion to elemental sulfur of sulfur dioxide initially contained in a gas mixture in which the SO.sub.2 was a minor component. Although this invention is useful in a variety of circumstances, for example, in processing SO.sub.2 contained in a gas mixture emanating from the roasting of sulfide ores, it is of especial utility in flue gas desulfurization (FGD).
It has long been known that when present in the atmosphere even in small concentrations, suflur dioxide is detrimental to the well-being of animal, aquatic and plant life, When converted to sulfuric acid by the oxygen and moisture in the air, which conversion takes place readily, sulfur dioxide is responsible for the corrosion of many materials of construction including steel and concrete.
Putting large amounts of SO.sub.2 into the air from fuel burning installations was prohibited in 1967. In that year Congress passed the Clean Air Act and amended it in 1970. As a result of this legislation the Environmental Protection Agency (EPA) has established standards which limits the emission of the principal pollutants contained in flue gas. These pollutants are particulates, sulfur dioxide and nitrogen oxides measured as nitrogen dioxide. Some of these standards are shown in Table 1.
TABLE 1 ______________________________________ Standards of Performance for New Fossil-Fired Steam Generators (Construction Commenced After August 17, 1971) Fuel Type-Maximum Quantity of Pollutants Permitted in Flue-Gas Pounds/Million BTU Particulates Sulfur Oxides Nitrogen Oxides ______________________________________ Solid .10 1.2 .70 Liquid .10 .8 .30 Gas -- -- .20 ______________________________________
The bulk of our heavy duty fuels i.e. butiminous coal and residual fuel oil contain appreciable percentages of sulfur--many containing more than 3% by weight. From the above figures it is evident that when 12,000 BTU per pound coal containing 0.8% sulfur is burned, the limits set by the EPA will be exceeded. To operate within EPA limits, installations burning large tonnages of economically priced fuel have had to resort to scrubbers to remove a large part of the SO.sub.2 contained in the flue gas leaving their combustion zones. Most of these scrubbers use lime or limtestone directly or indirectly to combine with the SO.sub.2 scrubbed out of the flue gas. The chief virtue of this practice is that the pollutant formed, a sludge composed mainly of calcium sulfite and calcium sulfate, is less objectionable than SO.sub.2 in air. These sludges are being impounded except where local circumstances allow them to be used as land-fill. When SO.sub.2 is scrubbed out of flue gas by a calcium compound, the resulting substance usually becomes a waste disposal problem. Disposing of this waste is a continuing source of expense in addition to the cost of the calcium compound consumed in the operation.